A wide range of industries such as, for example, electric utilities, power plants, oil refineries, off shore oil rigs, gas ethylene companies, chemical plants, coal mining operations, coal prep plants and transfer stations, gas pipelines, plastic manufacturers, granaries, etc. present very hazardous environments in which electrical equipment must be used. Because of these dangerous environments and because of the hazards imposed by the use of electrical equipment in them, various standards have been imposed by the National Electrical Code and by Underwriters Laboratories for the design of electrical equipment for hazardous areas.
The National Electrical Code (NEC) classifies hazardous areas in industrial environments depending upon the properties of the materials found in those environments. (See ANSI/NFPA-70, Article 500.) Class 1 hazardous environments are those containing inflammable gases and vapors. Class 2 hazardous areas are those containing inflammable dusts. Class 3 hazardous areas are those containing fibers and flyings. Within each of the aforementioned classes, environments are further grouped in accordance with the particular materials found there. Class 1, Group A, environments are those which include acetylene gas. For example, Class 1, Group A, environments are the most hazardous environments classified by the NEC and these require the most stringent specifications for electrical equipment. Because of the nature of acetylene gas, very little electrical equipment has ever been approved for use in Class 1, Group A, environments. In fact, no telephone equipment has ever been approved for use in such locations.
Class 1, Group B, environments are those including hydrogen gas or manufactured gases containing more than 30% hydrogen by volume. Class 1, Group C, environments are those containing carbon monoxide, hydrogen sulfide, crude oil, etc. Class 1, Group D, environments are those containing acetone, benzone, butyl, ethyl, methyl, propyl and isopropyl alcohols, gasoline, methane, styrene, toluene, vinyl chloride, etc. Telephone equipment has been approved for use in Class 1, Group B, C and D, environments but such equipment is very expensive.
Electrical equipment, including telephones, which is approved for use in Class 1 hazardous areas as classified by the NEC, without the use of approved enclosures, i.e., explosion-proof, purged and pressurized, is referred to as "intrinsically safe". Typical prior art explosion-proof telephones approved for use in Class 1, Groups B, C and D type hazardous environments have enclosures which are not air tight. The enclosures for such explosion-proof telephones, therefore, allow the entry of the surrounding hazardous atmosphere. Any arcing of the device within its enclosure can, therefore, cause ignition inside the enclosure. The enclosure must therefore be constructed to withstand and contain the resulting high pressures caused by the internal explosion. Such enclosures usually include heavy aluminum castings with wide, closely machined cover plates secured in place by many hold down bolts. Any pass through devices through the enclosure, such as push buttons or rotary motion switches, must also maintain certain clearances with a minimum length bushing or sleeve. Such enclosures are heavy and expensive to construct.
It would be desirable to provide a hazardous area telephone for use in Class 1, Group B, C and D, type hazardous environments as classified by the National Electrical Code which did not require the use of an explosion-proof housing of the type described above. It would be further desirable to provide an intrinsically safe telephone for use in Class 1, Group A, environments and it would be even more desirable to produce such a telephone which did not require an explosion-proof housing.
The design of an intrinsically safe telephone which meets Underwriter Laboratories (UL) requirements for installation in hazardous environments of the type classified by the National Electrical Code is not a simple matter.
In addition to meeting the constraints imposed by Underwriters Laboratories, it is also required that an intrinsically safe telephone meet the requirements of the Federal Communications Commission for interface with standard telephone networks. Thus, while an intrinsically safe telephone meeting the object of the present invention must meet the requirements of the NEC and of the Underwriters Laboratories for hazardous environments, it must also have standard line impedances and characteristics in order to interface with pre-existing telephone networks.
Accordingly, it is an object of the present invention to provide an intrinsically safe telephone which is capable of operating in all hazardous environments as defined by the National Electrical Code, which does so without the use of an expensive explosion-proof housing, which meets all of the requirements of Underwriters Laboratories for operation within hazardous environments and which is fully compatible with pre-existing telephone networks.
The difficulty with prior art telephones intended to be intrinsically safe has been the fact that such telephones require auxiliary power. Special cabling runs must be made in large plants to provide electrical power for operation of telephones in hazardous areas. These cable runs are costly. In addition, loss of electrical power to the plant results in a loss of telephone communications. Since telephones may be located in hazardous areas for use in emergency situations, it is particularly disadvantageous to lose telephone communication during a power outage. It would be particularly desirable to provide an intrinsically safe telephone for use in hazardous environments which did not require auxiliary power cabling.